The samples were from three brain regions in the cortex: the dorsolateral prefrontal cortex, the posterior superior temporal cortex and the occipital cortex. These parts of the brain play a role in social cognition, inhibition and abstract reasoning.

The researchers analyzed 11 samples from children with autism and 11 samples from typically developing children. They were looking to see whether the samples had developed into six distinct layers -- each with its own set of gene markers -- which should happen in prenatal development. Some of the genes they examined are linked to autism, including RORB, NEFL and PDE1A.

In 10 of the 11 samples from children with autism, the researchers found patches of tissue where some layers were missing, according to results published in Thursday's edition of the New England Journal of Medicine. In the 11 samples that were used as controls, only one had an abnormal patch.

“The fact that we were able to find these patches is remarkable, given that the cortex is roughly the size of the surface of a basketball, and we only examined pieces of tissue the size of a pencil eraser,” Ed Lein, a study coauthor from the Allen Institute for Brain Science in Seattle, said in a statement. “This suggests these abnormalities are quite pervasive across the surface of the cortex.”

And that is a sign that something is going wrong in the womb, since that’s where the six layers are formed.

“Our data are consistent with an early prenatal origin of autism or at least prenatal processes that may confer a predisposition to autism,” the researchers concluded in their study. However, the results don’t reveal the pathological mechanism that causes the disorder.